Introduction to the OAI

Metadata Harvesting Protocol

Hussein Suleman, hussein@vt.edu

Digital Library Research Laboratory

Virginia Tech

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1. Introduction

What is the OAI-MHP?

General System Strategy

Case study: NDLTD

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1.1. What is the OAI-MHP ?

What is the Metadata Harvesting Protocol?Protocol to transfer metadata from a source archive to a destination archive

• Any metadata

• In a continuous stream

• As simply as possible

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1.2. General System Strategy

Services

Metadata Harvesting

Document Model

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1.3. Case Study: NDLTD

Networked Digital Library of Theses and Dissertations

Multiple independent university-based collections of electronic documents

Virginia Tech

Rhodes U.

U.Waterloo

International

ETD

Library

OAI

Metadata

Harvesting

Protocol

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2. Definitions / Concepts

Basic PrinciplesWhat is an Open Archive?

Harvesting vs. Federation

Data and Service Providers

Underlying TechnologyHTTP and XML

Protocol PoliciesWhat is a record?

Multiplicity of Metadata

Sets

Datestamp, Harvesting and Flow Control

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2.1. What is an Open Archive ?

Any WWW-based system that can be accessed through the well-defined interface of the Open Archives Protocol for Metadata Harvesting… aka OAI-Compliant RepositoryNo implications for:

Physical storage of dataCost of dataMetadata and data formatsAccess control to server

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2.2. Harvesting vs Federation

Competing approaches to interoperabilityFederation is when services are run remotely on remote data (e.g. Federated searching)

Harvesting is when data/metadata is transferred from the remote source to the destination where the services are located (e.g. Union catalogues)

Federation requires more effort at each remote source but is easier for the local system and vice versa for harvesting

OAI currently focuses on harvesting

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2.3. Data and Service Providers

Data Providers refer to entities who possess data/metadata and are willing to share this with others (internally or externally) via well-defined OAI protocols (e.g. database servers)Service Providers are entities who harvest data from Data Providers in order to provide higher-level services to users (e.g. search engines)OAI uses these denotations for its client/server model (data=server, service=client)

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2.4. HTTP and XML

Metadata Harvesting Protocol is an almost stateless request/response protocol

Requests and responses are sent via the HTTP protocol

Requests are encoded as GET/POST operations

Responses are well-formed XML documents

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2.5. What is a record ?

A record refers to an independent XML structure that may be associated with digital or physical objects

Records are usually associated with metadata, not data

OAI advocates harvesting of records, which contain metadata and additional fields to support the harvesting operation

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2.6. Sample OAI Record

<record> <header> <identifier>oai:sigir:ws3</identifier> <datestamp>2001-08-13</datestamp> </header> <metadata> <dc> <title>OAI Workshop at SIGIR</title> <creator>Hussein Suleman</creator> <language>English</language> </dc> </metadata> <about> <metadataID>oai:sigir:ws3md</metadataID> </about></record>

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2.7. Multiplicity of Metadata

Multiple formats of metadata allowed

Dublin Core is mandatory

Any other format allowed as long as it has an XML encoding

E.g. MARC (Libraries), IMS (Education), ETDMS (Theses/Dissertations), RFC1807 (Bibliographies)

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2.8. Sets

Protocol mechanism to allow for harvesting of sub-collections

No well-defined semantics – depends completely on local data providers

May be defined by arrangement between data providers and service providers

E.g. Subject areas, years, author names, search queries

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2.9. Datestamps & Harvesting

Each record needs a datestamp that indicates its date of creation or modification

Dates are used to allow for harvesting by date range, thus allowing incremental and continuous transfer of metadata from a data provider to a service provider

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2.10. Flow Control

HTTP “retry-after” mechanism can be leveraged to support server-side delaying of a client’s request

Resumption Tokens can be used to return partial results – the client is issued with a token which may be presented to the server to receive more results

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3. Metadata Harvesting Protocol

Service RequestsIdentifyListMetadataFormatsListSetsGetRecordListIdentifiersListRecords

Metadata MultiplicityDate RangesResumption Tokens

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3.1. Identify

PurposeReturn general information about the archive and its policies

ParametersNone

Sample URLhttp://www.anarchive.org/cgi-bin/OAI?verb=Identify

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3.2. Identify - Response

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3.3. ListMetadataFormats

PurposeList metadata formats supported by the archive as well as their schema locations and namespaces

Parametersidentifier – for a specific record (O)

Sample URLhttp://www.anarchive.org/cgi-bin/OAI?verb=ListMetadataFormats

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3.4. ListMetadataFormats - Response

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3.5. ListSets

PurposeProvide a hierarchical listing of sets in which records may be organized

ParametersNone

Sample URLhttp://www.anarchive.org/cgi-bin/OAI?verb=ListSets

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3.6. ListSets – Response

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3.7. GetRecord

PurposeReturns the metadata for a single identifier in the form of an OAI record

Parametersidentifier – unique id for record (R)

metadataPrefix – metadata format (R)

Sample URLhttp://www.anarchive.org/cgi-bin/OAI?verb=GetRecord&identifier=oai:test:123&metadataPrefix=oai_dc

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3.8. GetRecord - Response

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3.9. ListIdentifiers

PurposeList all unique identifiers corresponding to records in the repository

Parametersfrom – start date (O)until – end date (O)set – set to harvest from (O)resumptionToken – flow control mechanism (X)

Sample URLhttp://www.anarchive.org/cgi-bin/OAI?verb=ListIdentifiers&set=All

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3.10. ListIdentifiers - Response

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3.11. ListRecords

PurposeRetrieves metadata for multiple records

Parametersfrom – start date (O)until – end date (O)set – set to harvest from (O)resumptionToken – flow control mechanism (X)metadataPrefix – metadata format (R)

Sample URLhttp://www.anarchive.org/cgi-bin/OAI?verb=ListRecord&metadataprefix=oai_dc&from=2001-01-01

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3.12. ListRecords - Response

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3.13. Metadata Multiplicity

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3.14. Date Ranges

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3.15. Resumption Token

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That’s All Folks !

The OAI Metadata Harvesting Protocol -

Communities and Services

Hussein Suleman, hussein@vt.edu

Digital Library Research Laboratory

Virginia Tech

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4. Service Providers

Harvesting 101/102/103

Scheduling

Tools

Repository Explorer

Case Study: ARC

Case Study: NDLTD

VTLS Virtua

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4.1. Harvesting 101

ListRecords (from=2000-09-12)

Response

ListRecords (resumptionToken=1)

Response

ListRecords (from=2000-09-13)

Response

resumption

Token=1

SERVICE

PROVIDER

DATA

PROVIDER

Set date=09-13

DAY ONE

DAY TWO

Set date=09-14...

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4.2. Harvesting 102

ListMetadataFormats

Response

ListIdentifiersResponse

GetRecord (id=1, prefix=oai_dc)

Response

Identifier:1

Identifier:2

Identifier:3

SERVICE

PROVIDER

DATA

PROVIDER

oai_dc

oai_rfc1807

GetRecord (id=2, prefix=oai_dc)

Response

...

record1

record2

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4.3. Harvesting 103

ListMetadataFormats (id=1)

Response

ListIdentifiersResponse

GetRecord (id=1, prefix=oai_dc)

Response

Identifier:1

Identifier:2

Identifier:3

SERVICE

PROVIDER

DATA

PROVIDER

oai_dc

oai_rfc1807

...

record1

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4.4. Scheduling

Problems:Granularity is coarse

Timezones are local for each site

Solutions:Overlap one day to compensate for granularity

Overlap one day or use remote times to compensate for timezones

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4.5. Tools

Check OAI website for sample codeXML parsers – depending on platform – check W3CXML Schema validators

Very few available – the reference version works but may not be easy to installIgnore validation if you can trust the source

Sample data providers – check the OAI website for a list of conformant public archives

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4.6. Repository Explorer

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4.7. Case Study: ARC

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4.8. Case Study: NDLTD

Virginia Tech U. OldenbergHumboldt U.

NDLTD ETD Union Catalog

VTLS Virtua MARIAN

Search/Browse Engines

Recommender Cross-Ref.

Other Services

…

…

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4.9. VTLS Virtua

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5. OAI Communities

Shared Metadata Formats

Shared semantics

Layering over OAI

Closed OAI networks

OAI within the DL

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5.1. Shared Metadata Formats

Use metadata formats accepted within a community to convey more specific information

ExamplesE-Print format (under development)

ETD-MS for theses and dissertations

VRA Core for multimedia

IMS Metadata for educational material

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5.2. Shared Semantics

Develop a shared understanding for the meanings of fields

ExamplesDeveloping controlled vocabularies for fields

Using specific fields for external links (OAI recommends using identifier in DC for this)

Choosing from among existing standards (like language names)

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5.3. Layering over OAI

Convert OAI records into more standard formats like MARC communications format

Collapse multiple requests into one to make harvesting easier

Name authority system (developed at OCLC) piggybacks name resolution over the OAI protocol

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5.4. Closed OAI networks

Data providers need not go public !

Within an organization, OAI can be used for data transfer among heterogeneous systems

More control over use, making global optimizations possible (like harvesting schedules and choice of metadata formats)

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5.5. OAI within the DL

Use the OAI protocol as the basis for components to communicate

ExamplesSearch Engines could use dynamic sets to correspond to search results

Browsing can be directed by sets

Reviews and Annotations can each be independent OAI data providers

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6. Now What ?

Reality Check

Links

More Links

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6.1. Reality Check

DO I REALLY WANT TO DO THIS?

Can I satisfy the requirements to be a data provider?

Do I want to be a service provider ?

Do I want to adopt and support this within my community ?

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6.2. Links

Open Archives Initiativehttp://www.openarchives.org

OAI Metadata Harvesting Protocolhttp://www.openarchives.org/OAI/openarchivesprotocol.htm

Virginia Tech DLRL OAI Projectshttp://www.dlib.vt.edu/projects/OAI/

Repository Explorerhttp://purl.org/net/oai_explorer

NDLTDhttp://www.ndltd.org

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6.3. More Links

ARC Cross-Archive Search Servicehttp://arc.cs.odu.edu/

XML Schema Validatorhttp://www.w3.org/2001/03/webdata/xsv

Dublin Core Metadata Initiativehttp://www.dublincore.org

E-Prints DL-in-a-boxhttp://www.eprints.org

XML Tools at W3Chttp://www.w3.org/XML/#software

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That’s All Folks !